User-preferred reproduction of out-of-gamut spot colors

ABSTRACT

A method for reproducing an out-of-gamut spot color includes determining a color gamut for a color printer, and specifying a spot color by color coordinates in a three-dimensional color space. A first target color is determined corresponding to a color having a minimum color difference to the specified spot color, and a second target color is determined corresponding to a color on the color gamut surface having a hue value equal to a hue value of the specified spot color. A path is defined on the color gamut surface connecting the first target color and the second target color, wherein a control parameter is used to specify a relative position along the defined path. A user interface is provided enabling a user to adjust the control parameter to specify an aim color.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 63/273,248, filed Oct. 29, 2021, which is incorporatedherein by reference in its entirety.

Reference is made to commonly assigned, co-pending U.S. patentapplication Ser. No. ______/______ (Docket K002377), entitled:“Reproducing out-of-gamut spot colors on a color printer,” by C.-H. Kuoet al., which is incorporated herein by reference.

FIELD OF THE INVENTION

This invention pertains to the field of electrographic printing and moreparticularly to a method for reproducing out-of-gamut spot colors.

BACKGROUND OF THE INVENTION

Electrophotography is a useful process for printing images on a receiver(or “imaging substrate”), such as a piece or sheet of paper or anotherplanar medium (e.g., glass, fabric, metal, or other objects) as will bedescribed below. In this process, an electrostatic latent image isformed on a photoreceptor by uniformly charging the photoreceptor andthen discharging selected areas of the uniform charge to yield anelectrostatic charge pattern corresponding to the desired image (i.e., a“latent image”).

After the latent image is formed, charged toner particles are broughtinto the vicinity of the photoreceptor and are attracted to the latentimage to develop the latent image into a toner image. Note that thetoner image may not be visible to the naked eye depending on thecomposition of the toner particles (e.g., clear toner).

After the latent image is developed into a toner image on thephotoreceptor, a suitable receiver is brought into juxtaposition withthe toner image. A suitable electric field is applied to transfer thetoner particles of the toner image to the receiver to form the desiredprint image on the receiver. The imaging process is typically repeatedmany times with reusable photoreceptors.

The receiver is then removed from its operative association with thephotoreceptor and subjected to heat or pressure to permanently fix(i.e., “fuse”) the print image to the receiver. Plural print images(e.g., separation images of different colors) can be overlaid on thereceiver before fusing to form a multicolor print image on the receiver.

As the digital printing technology is beginning to gain a foothold inpackage printing, it is imperative for the digital printing system tosatisfactorily match any predefined spot color. While it is moreeconomical for the traditional high-volume package printing process toutilize premixed inks that match the intended custom spot colors, thissolution is generally not suitable for short-run package print jobs.Typically, the printing systems used for such applications (e.g.,electrophotographic printing systems) use a predefined set of colorants(e.g., cyan, magenta, yellow and black toners), and it would not beeconomical to change the colorants for short-run print jobs. Many spotcolors do not fall within the color gamut associated with the predefinedset of colorants. As a result, it is necessary to reproduce the spotcolors using a color which falls within the color gamut. Thedetermination of the optimal in-gamut colors for reproducingout-of-gamut spot colors is not a trivial process. Various“gamut-mapping algorithms” have been proposed for mapping out-of-gamutcolors to appropriate colors on the surface of the device color gamut.One prior art gamut mapping algorithm finds the in-gamut color havingthe smallest color difference to the out-of-gamut color. Such algorithmsare prone to introducing objectionable hue shifts. Another prior artgamut mapping algorithm operates by clipping the chroma to the colorgamut while maintaining the hue and lightness of the out-of-gamut color.Such algorithms are prone to introducing objectionable chromareductions. As a result of these deficiencies, many users choose tomanually determine a preferred aim color for out-of-gamut spot colors.This typically requires a time-consuming iterative process to determinethe preferred color reproduction given the complex shape of typicalcolor gamuts and the fact that a plurality of control parameters (e.g.,hue and lightness) must be adjusted to move around on the color gamutsurface. This process needs to be repeated every time a new spot coloris introduced. Furthermore, the preferred aim color may becustomer-dependent so that the preferred aim color determined for onecustomer may not be the optimal solution for a different customer.

There remains a need for a user-friendly method to determine preferredaim colors for out-of-gamut spot colors in a color printing system.

SUMMARY OF THE INVENTION

The present invention represents a method for reproducing anout-of-gamut spot color on a color printer, including:

determining a color gamut for the color printer, the color gamut beingdefined by a color gamut surface in a three-dimensional color spacerepresenting the colors that can be printed by the color printer;

specifying a spot color by color coordinates in the three-dimensionalcolor space, wherein the spot color is outside of the color gamutsurface;

determining a first target color corresponding to a color on the colorgamut surface having a minimum color difference to the specified spotcolor;

determining a second target color corresponding to a color on the colorgamut surface having a hue value equal to a hue value of the specifiedspot color;

defining a path on the color gamut surface connecting the first targetcolor and the second target color, wherein a control parameter having acontrol parameter value is used to specify a relative position along thedefined path; and

providing a user interface enabling a user to adjust the controlparameter to specify an aim color along the defined path for reproducingthe spot color using the color printer.

This invention has the advantage that an aim color corresponding to apreferred reproduction of a spot color can be determined using only asingle control parameter.

It has the additional advantage that a control parameter predictionfunction can be used to automatically compute a control parameter valuethat can be used to determine an initial preferred aim color.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational cross-section of an electrophotographic printersuitable for use with various embodiments;

FIG. 2 is an elevational cross-section of one printing module of theelectrophotographic printer of FIG. 1 ;

FIG. 3 shows a processing path for producing a printed image using apre-processing system coupled to a print engine;

FIGS. 4A-4B illustrate the CIELAB color space;

FIG. 5 shows an exemplary spot color which is outside the color gamut ofa printing device;

FIG. 6 shows a flowchart for an exemplary method for determining apreferred aim color for reproducing a spot color in accordance with anembodiment of the invention;

FIG. 7 illustrates a path formed on the color gamut surface between twotarget colors;

FIGS. 8A-8C illustrate different user interface examples for use withthe method of FIG. 6 ; and

FIG. 9 is a flowchart of a method for determining a control parameterprediction function in accordance with an exemplary embodiment.

It is to be understood that the attached drawings are for purposes ofillustrating the concepts of the invention and may not be to scale.Identical reference numerals have been used, where possible, todesignate identical features that are common to the figures.

DETAILED DESCRIPTION OF THE INVENTION

The invention is inclusive of combinations of the embodiments describedherein. References to “a particular embodiment” and the like refer tofeatures that are present in at least one embodiment of the invention.Separate references to “an embodiment” or “particular embodiments” orthe like do not necessarily refer to the same embodiment or embodiments;however, such embodiments are not mutually exclusive, unless soindicated, or as are readily apparent to one of skill in the art. Theuse of singular or plural in referring to the “method” or “methods” andthe like is not limiting. It should be noted that, unless otherwiseexplicitly noted or required by context, the word “or” is used in thisdisclosure in a non-exclusive sense.

As used herein, “sheet” is a discrete piece of media, such as receivermedia for an electrophotographic printer (described below). Sheets havea length and a width. Sheets are folded along fold axes (e.g.,positioned in the center of the sheet in the length dimension, andextending the full width of the sheet). The folded sheet contains two“leaves,” each leaf being that portion of the sheet on one side of thefold axis. The two sides of each leaf are referred to as “pages.” “Face”refers to one side of the sheet, whether before or after folding.

As used herein, “toner particles” are particles of one or morematerial(s) that are transferred by an electrophotographic (EP) printerto a receiver to produce a desired effect or structure (e.g., a printimage, texture, pattern, or coating) on the receiver. Toner particlescan be ground from larger solids, or chemically prepared (e.g.,precipitated from a solution of a pigment and a dispersant using anorganic solvent), as is known in the art. Toner particles typically havea range of diameters (e.g., less than 8 μm, on the order of 10-15 μm, upto approximately 30 μm, or larger), where “diameter” preferably refersto the volume-weighted median diameter, as determined by a device suchas a Coulter Multisizer.

“Toner” refers to a material or mixture that contains toner particles,and that can be used to form an image, pattern, or coating whendeposited on an imaging member including a photoreceptor, aphotoconductor, or an electrostatically-charged or magnetic surface.Toner can be transferred from the imaging member to a receiver. Toner isalso referred to in the art as marking particles, dry ink, or developer,but note that herein “developer” is used differently, as describedbelow. Toner can be a dry mixture of particles or a suspension ofparticles in a liquid toner base.

As mentioned already, toner includes toner particles; it can alsoinclude other types of particles. The particles in toner can be ofvarious types and have various properties. Such properties can includeabsorption of incident electromagnetic radiation (e.g., particlescontaining colorants such as dyes or pigments), absorption of moistureor gasses (e.g., desiccants or getters), suppression of bacterial growth(e.g., biocides, particularly useful in liquid-toner systems), adhesionto the receiver (e.g., binders), electrical conductivity or low magneticreluctance (e.g., metal particles), electrical resistivity, texture,gloss, magnetic remanence, florescence, resistance to etchants, andother properties of additives known in the art.

In single-component or mono-component development systems, “developer”refers to toner alone. In these systems, none, some, or all of theparticles in the toner can themselves be magnetic. However, developer ina mono-component system does not include magnetic carrier particles. Indual-component, two-component, or multi-component development systems,“developer” refers to a mixture including toner particles and magneticcarrier particles, which can be electrically-conductive or-non-conductive. Toner particles can be magnetic or non-magnetic. Thecarrier particles can be larger than the toner particles (e.g., 15-20 μmor 20-300 μm in diameter). A magnetic field is used to move thedeveloper in these systems by exerting a force on the magnetic carrierparticles. The developer is moved into proximity with an imaging memberor transfer member by the magnetic field, and the toner or tonerparticles in the developer are transferred from the developer to themember by an electric field, as will be described further below. Themagnetic carrier particles are not intentionally deposited on the memberby action of the electric field; only the toner is intentionallydeposited. However, magnetic carrier particles, and other particles inthe toner or developer, can be unintentionally transferred to an imagingmember. Developer can include other additives known in the art, such asthose listed above for toner. Toner and carrier particles can besubstantially spherical or non-spherical.

The electrophotographic process can be embodied in devices includingprinters, copiers, scanners, and facsimiles, and analog or digitaldevices, all of which are referred to herein as “printers.” Variousembodiments described herein are useful with electrostatographicprinters such as electrophotographic printers that employ tonerdeveloped on an electrophotographic receiver, and ionographic printersand copiers that do not rely upon an electrophotographic receiver.Electrophotography and ionography are types of electrostatography(printing using electrostatic fields), which is a subset ofelectrography (printing using electric fields). The present inventioncan be practiced using any type of electrographic printing system,including electrophotographic and ionographic printers.

A digital reproduction printing system (“printer”) typically includes adigital front-end processor (DFE), a print engine (also referred to inthe art as a “marking engine”) for applying toner to the receiver, andone or more post-printing finishing system(s) (e.g., a UV coatingsystem, a glosser system, or a laminator system). A printer canreproduce pleasing black-and-white or color images onto a receiver. Aprinter can also produce selected patterns of toner on a receiver, whichpatterns (e.g., surface textures) do not correspond directly to avisible image.

In an embodiment of an electrophotographic modular printing machineuseful with various embodiments (e.g., the NEXFINITY Digital Pressmanufactured by Eastman Kodak Company of Rochester, N.Y.) color-tonerprint images are made in a plurality of color imaging modules arrangedin tandem, and the print images are successively electrostaticallytransferred to a receiver adhered to a transport web moving through themodules. Colored toners include colorants, (e.g., dyes or pigments)which absorb specific wavelengths of visible light. Commercial machinesof this type typically employ intermediate transfer members in therespective modules for transferring visible images from thephotoreceptor and transferring print images to the receiver. In otherelectrophotographic printers, each visible image is directly transferredto a receiver to form the corresponding print image.

Electrophotographic printers having the capability to also deposit cleartoner using an additional imaging module are also known. The provisionof a clear-toner overcoat to a color print is desirable for providingfeatures such as protecting the print from fingerprints, reducingcertain visual artifacts or providing desired texture or surface finishcharacteristics. Clear toner uses particles that are similar to thetoner particles of the color development stations but without coloredmaterial (e.g., dye or pigment) incorporated into the toner particles.However, a clear-toner overcoat can add cost and reduce color gamut ofthe print; thus, it is desirable to provide for operator/user selectionto determine whether or not a clear-toner overcoat will be applied tothe entire print. A uniform layer of clear toner can be provided. Alayer that varies inversely according to heights of the toner stacks canalso be used to establish level toner stack heights. The respectivecolor toners are deposited one upon the other at respective locations onthe receiver and the height of a respective color toner stack is the sumof the toner heights of each respective color. Uniform stack heightprovides the print with a more even or uniform gloss.

FIGS. 1 and 2 are elevational cross-sections showing portions of atypical electrophotographic printer 100 useful with various embodiments.Printer 100 is adapted to produce images, such as single-color images(i.e., monochrome images), or multicolor images such as CMYK, orpentachrome (five-color) images, on a receiver. Multicolor images arealso known as “multi-component” images. One embodiment involves printingusing an electrophotographic print engine having five sets ofsingle-color image-producing or image-printing stations or modulesarranged in tandem, but more or less than five colors can be combined ona single receiver. Other electrophotographic writers or printerapparatus can also be included. Various components of printer 100 areshown as rollers; other configurations are also possible, includingbelts. Referring to FIG. 1 , printer 100 is an electrophotographicprinting apparatus having a number of tandemly-arrangedelectrophotographic image-forming printing subsystems 31, 32, 33, 34,35, also known as electrophotographic imaging subsystems. Each printingsubsystem 31, 32, 33, 34, 35 produces a single-color toner image fortransfer using a respective transfer subsystem 50 (for clarity, only oneis labeled) to a receiver 42 successively moved through the modules. Insome embodiments one or more of the printing subsystem 31, 32, 33, 34,35 can print a colorless toner image, which can be used to provide aprotective overcoat or tactile image features. Receiver 42 istransported from supply unit 40, which can include active feedingsubsystems as known in the art, into printer 100 using a transport web81. In various embodiments, the visible image can be transferreddirectly from an imaging roller to a receiver, or from an imaging rollerto one or more transfer roller(s) or belt(s) in sequence in transfersubsystem 50, and then to receiver 42. Receiver 42 is, for example, aselected section of a web or a cut sheet of a planar receiver media suchas paper or transparency film.

In the illustrated embodiments, each receiver 42 can have up to fivesingle-color toner images transferred in registration thereon during asingle pass through the five printing subsystems 31, 32, 33, 34, 35 toform a pentachrome image. As used herein, the term “pentachrome” impliesthat in a print image, combinations of various of the five colors arecombined to form other colors on the receiver at various locations onthe receiver, and that all five colors participate to form processcolors in at least some of the subsets. That is, each of the five colorsof toner can be combined with toner of one or more of the other colorsat a particular location on the receiver to form a color different thanthe colors of the toners combined at that location. In an exemplaryembodiment, printing subsystem 31 forms black (K) print images, printingsubsystem 32 forms yellow (Y) print images, printing subsystem 33 formsmagenta (M) print images, and printing subsystem 34 forms cyan (C) printimages.

Printing subsystem 35 can form a red, blue, green, or other fifth printimage, including an image formed from a clear toner (e.g., one lackingpigment). The four subtractive primary colors, cyan, magenta, yellow,and black, can be combined in various combinations of subsets thereof toform a representative spectrum of colors. The color gamut of a printer(i.e., the range of colors that can be produced by the printer) isdependent upon the materials used and the process used for forming thecolors. The fifth color can therefore be added to improve the colorgamut. In addition to adding to the color gamut, the fifth color canalso be a specialty color toner or spot color, such as for makingproprietary logos or colors that cannot be produced with only CMYKcolors (e.g., metallic, fluorescent, or pearlescent colors), or a cleartoner or tinted toner. Tinted toners absorb less light than theytransmit, but do contain pigments or dyes that move the hue of lightpassing through them towards the hue of the tint. For example, ablue-tinted toner coated on white paper will cause the white paper toappear light blue when viewed under white light, and will cause yellowsprinted under the blue-tinted toner to appear slightly greenish underwhite light.

Receiver 42 a is shown after passing through printing subsystem 31.Print image 38 on receiver 42 a includes unfused toner particles.Subsequent to transfer of the respective print images, overlaid inregistration, one from each of the respective printing subsystems 31,32, 33, 34, 35, receiver 42 a is advanced to a fuser module 60 (i.e., afusing or fixing assembly) to fuse the print image 38 to the receiver 42a. Transport web 81 transports the print-image-carrying receivers to thefuser module 60, which fixes the toner particles to the respectivereceivers, generally by the application of heat and pressure. Thereceivers are serially detacked from the transport web 81 to permit themto feed cleanly into the fuser module 60. The transport web 81 is thenreconditioned for reuse at cleaning station 86 by cleaning andneutralizing the charges on the opposed surfaces of the transport web81. A mechanical cleaning station (not shown) for scraping or vacuumingtoner off transport web 81 can also be used independently or withcleaning station 86. The mechanical cleaning station can be disposedalong the transport web 81 before or after cleaning station 86 in thedirection of rotation of transport web 81.

In the illustrated embodiment, the fuser module 60 includes a heatedfusing roller 62 and an opposing pressure roller 64 that form a fusingnip 66 therebetween. In an embodiment, fuser module 60 also includes arelease fluid application substation 68 that applies release fluid,e.g., silicone oil, to fusing roller 62. Alternatively, wax-containingtoner can be used without applying release fluid to the fusing roller62. Other embodiments of fusers, both contact and non-contact, can beemployed. For example, solvent fixing uses solvents to soften the tonerparticles so they bond with the receiver. Photoflash fusing uses shortbursts of high-frequency electromagnetic radiation (e.g., ultravioletlight) to melt the toner. Radiant fixing uses lower-frequencyelectromagnetic radiation (e.g., infrared light) to more slowly melt thetoner. Microwave fixing uses electromagnetic radiation in the microwaverange to heat the receivers (primarily), thereby causing the tonerparticles to melt by heat conduction, so that the toner is fixed to thereceiver.

The fused receivers (e.g., receiver 42 b carrying fused image 39) aretransported in series from the fuser module 60 along a path either to anoutput tray 69, or back to printing subsystems 31, 32, 33, 34, 35 toform an image on the backside of the receiver (i.e., to form a duplexprint). Receivers 42 b can also be transported to any suitable outputaccessory. For example, an auxiliary fuser or glossing assembly canprovide a clear-toner overcoat. Printer 100 can also include multiplefuser modules 60 to support applications such as overprinting, as knownin the art.

In various embodiments, between the fuser module 60 and the output tray69, receiver 42 b passes through a finisher 70. Finisher 70 performsvarious paper-handling operations, such as folding, stapling,saddle-stitching, collating, and binding.

Printer 100 includes main printer apparatus logic and control unit (LCU)99, which receives input signals from various sensors associated withprinter 100 and sends control signals to various components of printer100. LCU 99 can include a microprocessor incorporating suitable look-uptables and control software executable by the LCU 99. It can alsoinclude a field-programmable gate array (FPGA), programmable logicdevice (PLD), programmable logic controller (PLC) (with a program in,e.g., ladder logic), microcontroller, or other digital control system.LCU 99 can include memory for storing control software and data. In someembodiments, sensors associated with the fuser module 60 provideappropriate signals to the LCU 99. In response to the sensor signals,the LCU 99 issues command and control signals that adjust the heat orpressure within fusing nip 66 and other operating parameters of fusermodule 60. This permits printer 100 to print on receivers of variousthicknesses and surface finishes, such as glossy or matte.

FIG. 2 shows additional details of printing subsystem 31, which isrepresentative of printing subsystems 32, 33, 34, and 35 (FIG. 1 ).Photoreceptor 206 of imaging member 111 includes a photoconductive layerformed on an electrically conductive substrate. The photoconductivelayer is an insulator in the substantial absence of light so thatelectric charges are retained on its surface. Upon exposure to light,the charge is dissipated. In various embodiments, photoreceptor 206 ispart of, or disposed over, the surface of imaging member 111, which canbe a plate, drum, or belt. Photoreceptors can include a homogeneouslayer of a single material such as vitreous selenium or a compositelayer containing a photoconductor and another material. Photoreceptors206 can also contain multiple layers.

Charging subsystem 210 applies a uniform electrostatic charge tophotoreceptor 206 of imaging member 111. In an exemplary embodiment,charging subsystem 210 includes a wire grid 213 having a selectedvoltage. Additional necessary components provided for control can beassembled about the various process elements of the respective printingsubsystems. Meter 211 measures the uniform electrostatic charge providedby charging subsystem 210.

An exposure subsystem 220 is provided for selectively modulating theuniform electrostatic charge on photoreceptor 206 in an image-wisefashion by exposing photoreceptor 206 to electromagnetic radiation toform a latent electrostatic image. The uniformly-charged photoreceptor206 is typically exposed to actinic radiation provided by selectivelyactivating particular light sources in an LED array or a laser deviceoutputting light directed onto photoreceptor 206. In embodiments usinglaser devices, a rotating polygon (not shown) is sometimes used to scanone or more laser beam(s) across the photoreceptor in the fast-scandirection. One pixel site is exposed at a time, and the intensity orduty cycle of the laser beam is varied at each dot site. In embodimentsusing an LED array, the array can include a plurality of LEDs arrangednext to each other in a linear array extending in a cross-trackdirection such that all dot sites in one row of dot sites on thephotoreceptor can be selectively exposed simultaneously, and theintensity or duty cycle of each LED can be varied within a line exposuretime to expose each pixel site in the row during that line exposuretime.

As used herein, an “engine pixel” is the smallest addressable unit onphotoreceptor 206 which the exposure subsystem 220 (e.g., the laser orthe LED) can expose with a selected exposure different from the exposureof another engine pixel. Engine pixels can overlap (e.g., to increaseaddressability in the slow-scan direction). Each engine pixel has acorresponding engine pixel location, and the exposure applied to theengine pixel location is described by an engine pixel level.

The exposure subsystem 220 can be a write-white or write-black system.In a write-white or “charged-area-development” system, the exposuredissipates charge on areas of photoreceptor 206 to which toner shouldnot adhere. Toner particles are charged to be attracted to the chargeremaining on photoreceptor 206. The exposed areas therefore correspondto white areas of a printed page. In a write-black or “discharged-areadevelopment” system, the toner is charged to be attracted to a biasvoltage applied to photoreceptor 206 and repelled from the charge onphotoreceptor 206. Therefore, toner adheres to areas where the charge onphotoreceptor 206 has been dissipated by exposure. The exposed areastherefore correspond to black areas of a printed page.

In the illustrated embodiment, meter 212 is provided to measure thepost-exposure surface potential within a patch area of a latent imageformed from time to time in a non-image area on photoreceptor 206. Othermeters and components can also be included (not shown).

A development station 225 includes toning shell 226, which can berotating or stationary, for applying toner of a selected color to thelatent image on photoreceptor 206 to produce a developed image onphotoreceptor 206 corresponding to the color of toner deposited at thisprinting subsystem 31. Development station 225 is electrically biased bya suitable respective voltage to develop the respective latent image,which voltage can be supplied by a power supply (not shown). Developeris provided to toning shell 226 by a supply system (not shown) such as asupply roller, auger, or belt. Toner is transferred by electrostaticforces from development station 225 to photoreceptor 206. These forcescan include Coulombic forces between charged toner particles and thecharged electrostatic latent image, and Lorentz forces on the chargedtoner particles due to the electric field produced by the bias voltages.

In some embodiments, the development station 225 employs a two-componentdeveloper that includes toner particles and magnetic carrier particles.The exemplary development station 225 includes a magnetic core 227 tocause the magnetic carrier particles near toning shell 226 to form a“magnetic brush,” as known in the electrophotographic art. Magnetic core227 can be stationary or rotating, and can rotate with a speed anddirection the same as or different than the speed and direction oftoning shell 226. Magnetic core 227 can be cylindrical ornon-cylindrical, and can include a single magnet or a plurality ofmagnets or magnetic poles disposed around the circumference of magneticcore 227. Alternatively, magnetic core 227 can include an array ofsolenoids driven to provide a magnetic field of alternating direction.Magnetic core 227 preferably provides a magnetic field of varyingmagnitude and direction around the outer circumference of toning shell226. Development station 225 can also employ a mono-component developercomprising toner, either magnetic or non-magnetic, without separatemagnetic carrier particles.

Transfer subsystem 50 includes transfer backup member 113, andintermediate transfer member 112 for transferring the respective printimage from photoreceptor 206 of imaging member 111 through a firsttransfer nip 201 to surface 216 of intermediate transfer member 112, andthence to a receiver 42 which receives respective toned print images 38from each printing subsystem in superposition to form a composite imagethereon. The print image 38 is, for example, a separation of one color,such as cyan. Receiver 42 is transported by transport web 81. Transferto a receiver is effected by an electrical field provided to transferbackup member 113 by power source 240, which is controlled by LCU 99.Receiver 42 can be any object or surface onto which toner can betransferred from imaging member 111 by application of the electricfield. In this example, receiver 42 is shown prior to entry into asecond transfer nip 202, and receiver 42 a is shown subsequent totransfer of the print image 38 onto receiver 42 a.

In the illustrated embodiment, the toner image is transferred from thephotoreceptor 206 to the intermediate transfer member 112, and fromthere to the receiver 42. Registration of the separate toner images isachieved by registering the separate toner images on the receiver 42, asis done with the NexPress 2100. In some embodiments, a single transfermember is used to sequentially transfer toner images from each colorchannel to the receiver 42. In other embodiments, the separate tonerimages can be transferred in register directly from the photoreceptor206 in the respective printing subsystem 31, 32, 33, 34, 25 to thereceiver 42 without using a transfer member. Either transfer process issuitable when practicing this invention. An alternative method oftransferring toner images involves transferring the separate tonerimages, in register, to a transfer member and then transferring theregistered image to a receiver.

LCU 99 sends control signals to the charging subsystem 210, the exposuresubsystem 220, and the respective development station 225 of eachprinting subsystem 31, 32, 33, 34, 35 (FIG. 1 ), among other components.Each printing subsystem can also have its own respective controller (notshown) coupled to LCU 99.

Various finishing systems can be used to apply features such asprotection, glossing, or binding to the printed images. The finishingsystem scan be implemented as an integral components of the printer 100,or can include one or more separate machines through which the printedimages are fed after they are printed.

FIG. 3 shows a processing path that can be used to produce a printedimage 450 with a print engine 370 in accordance with embodiments of theinvention. A pre-processing system 305 is used to process a pagedescription file 300 to provide image data 350 that is in a form that isready to be printed by the print engine 370. In an exemplaryconfiguration, the pre-processing system 305 includes a digital frontend (DFE) 310 and an image processing module 330. The pre-processingsystem 305 can be a part of printer 100 (FIG. 1 ), or may be a separatesystem which is remote from the printer 100. The DFE 310 and an imageprocessing module 330 can each include one or more suitably-programmedcomputer or logic devices adapted to perform operations appropriate toprovide the image data 350.

The DFE 310 receives page description files 300 which define the pagesthat are to be printed. The page description files 300 can be in anyappropriate format (e.g., the well-known Postscript command file formator the PDF file format) that specifies the content of a page in terms oftext, graphics and image objects. The image objects are typicallyprovided by input devices such as scanners, digital cameras or computergenerated graphics systems. The page description file 300 can alsospecify invisible content such as specifications of texture, gloss orprotective coating patterns.

The DFE 310 rasterizes the page description file 300 into image bitmapsfor the print engine to print. The DFE 310 can include variousprocessors, such as a raster image processor (RIP) 315, a colortransform processor 320 and a compression processor 325. It can alsoinclude other processors not shown in FIG. 3 , such as an imagepositioning processor or an image storage processor. In someembodiments, the DFE 310 enables a human operator to set up parameterssuch as layout, font, color, media type or post-finishing options.

The RIP 315 rasterizes the objects in the page description file 300 intoan image bitmap including an array of image pixels at an imageresolution that is appropriate for the print engine 370. For text orgraphics objects the RIP 315 will create the image bitmap based on theobject definitions. For image objects, the RIP 315 will resample theimage data to the desired image resolution.

The color transform processor 320 will transform the image data to thecolor space required by the print engine 370, providing colorseparations for each of the color channels (e.g., CMYK). For cases wherethe print engine 370 includes one or more additional colors (e.g., red,blue, green, gray or clear), the color transform processor 320 will alsoprovide color separations for each of the additional color channels. Theobjects defined in the page description file 300 can be in anyappropriate input color space such as RGB, CIELAB, PCS LAB or CMYK. Insome cases, different objects may be defined using different colorspaces. The color transform processor 320 applies an appropriate colortransform to convert the objects to the device-dependent color space ofthe print engine 370. Methods for creating such color transforms arewell-known in the color management art, and any such method can be usedin accordance with the present invention. Typically, the colortransforms are defined using color management profiles that includemulti-dimensional look-up tables. Input color profiles are used todefine a relationship between the input color space and a profileconnection space (PCS) defined for a color management system (e.g., thewell-known ICC PCS associated with the ICC color management system).Output color profiles define a relationship between the PCS and thedevice-dependent output color space for the printer 100. The colortransform processor 320 transforms the image data using the colormanagement profiles. Typically, the output of the color transformprocessor 320 will be a set of color separations including an array ofpixels for each of the color channels of the print engine 370 stored inmemory buffers.

The processing applied in digital front end 310 can also include otheroperations not shown in FIG. 3 . For example, in some configurations,the DFE 310 can apply the halo correction process described incommonly-assigned U.S. Pat. No. 9,147,232 to Kuo entitled “Reducing haloartifacts in electrophotographic printing systems,” which isincorporated herein by reference.

The image data provided by the digital front end 310 is sent to theimage processing module 330 for further processing. In order to reducethe time needed to transmit the image data, a compressor processor 325is typically used to compress the image data using an appropriatecompression algorithm. In some cases, different compression algorithmscan be applied to different portions of the image data. For example, alossy compression algorithm (e.g., the well-known JPEG algorithm) can beapplied to portions of the image data including image objects, and alossless compression algorithm can be applied to portions of the imagedata including binary text and graphics objects. The compressed imagevalues are then transmitted over a data link to the image processingmodule 330, where they are decompressed using a decompression processor335 which applies corresponding decompression algorithms to thecompressed image data.

A halftone processor 340 is used to apply a halftoning process to theimage data. The halftone processor 340 can apply any appropriatehalftoning process known in the art. Within the context of the presentdisclosure, halftoning processes are applied to a continuous-tone imageto provide an image having a halftone dot structure appropriate forprinting using the printer module 435. The output of the halftoning canbe a binary image or a multi-level image. In an exemplary configuration,the halftone processor 340 applies the halftoning process described incommonly assigned U.S. Pat. No. 7,830,569 to Tai et al., entitled“Multilevel halftone screen and sets thereof,” which is incorporatedherein by reference. For this halftoning process, a three-dimensionalhalftone screen is provided that includes a plurality of planes, eachcorresponding to one or more intensity levels of the input image data.Each plane defines a pattern of output exposure intensity valuescorresponding to the desired halftone pattern. The halftoned pixelvalues are multi-level values at the bit depth appropriate for the printengine 370.

The image enhancement processor 345 can apply a variety of imageprocessing operations. For example, an image enhancement processor 345can be used to apply various image enhancement operations. In someconfigurations, the image enhancement processor 345 can apply analgorithm that modifies the halftone process in edge regions of theimage (see U.S. Pat. No. 7,079,281, entitled “Edge enhancement processorand method with adjustable threshold setting” and U.S. Pat. No.7,079,287 entitled “Edge enhancement of gray level images,” both to Nget al., and both of which are incorporated herein by reference).

The pre-processing system 305 provides the image data 350 to the printengine 370, where it is printed to provide the printed image 450. Thepre-processing system 305 can also provide various signals to the printengine 370 to control the timing at which the image data 350 is printedby the print engine 370. For example, the pre-processing system 305 cansignal the print engine 370 to start printing when a sufficient numberof lines of image data 350 have been processed and buffered to ensurethat the pre-processing system 305 will be capable of keeping up withthe rate at which the print engine 370 can print the image data 350.

A data interface 405 in the print engine 370 receives the data from thepre-processing system 305. The data interface 405 can use any type ofcommunication protocol known in the art, such as standard Ethernetnetwork connections. A printer module controller 430 controls theprinter module 435 in accordance with the received image data 350. In anexemplary configuration, the printer module 435 can be the printer 100of FIG. 1 , which includes a plurality of individual electrophotographicprinting subsystems 31, 32, 33, 34, 35 for each of the color channels.For example, the printer module controller 430 can provide appropriatecontrol signals to activate light sources in the exposure subsystem 220(FIG. 2 ) to expose the photoreceptor 206 with an exposure pattern. Insome configurations, the printer module controller 430 can apply variousimage enhancement operations to the image data. For example, analgorithm can be applied to compensate for various sources ofnon-uniformity in the printer 100 (e.g., streaks formed in the chargingsubsystem 210, the exposure subsystem 220, the development station 225or the fuser module 60). One such compensation algorithm is described incommonly-assigned U.S. Pat. No. 8,824,907 to Kuo et al., entitled“Electrophotographic printing with column-dependent tonescaleadjustment,” which is incorporated herein by reference.

In some cases, the printing system can also include an image capturesystem 440. The image capture system can be used for purposes such assystem calibration. The image capture system 440 can use any appropriateimage capture technology such as a digital scanner system, or a digitalcamera system. The image capture system 440 can be integrated into theprinting system, or can be a separate system which is in communicationwith the printing system.

In the configuration of FIG. 3 , the pre-processing system 305 istightly coupled to the print engine 370 in that it supplies image data350 in a state which is matched to the printer resolution and thehalftoning state required for the printer module 435. In otherconfigurations, the print engine can be designed to be adaptive to thecharacteristics of different pre-processing systems 305 as is describedin commonly-assigned, co-pending U.S. Pat. No. 10,062,017 to Kuo et al.,entitled “Print engine with adaptive processing,” which is incorporatedherein by reference.

As discussed earlier, the color transform processor 320 (FIG. 3 ) willtransform the image data to the color space required by the print engine370, providing color separations for each of the color channels (e.g.,CMYK). The objects defined in the page description file 300 can includeraster image data in any appropriate input color space such as RGB,CIELAB, PCS LAB or CMYK. In some cases, the page description file 300can also include objects (e.g., graphics and text) specified as a named“spot color.” Such spot colors can be used to identify the colors to beused for content such as the company logos and product packaging.Examples of systems for naming spot colors would include the well-knownPantone Matching System, which is a proprietary color naming system foridentifying colors used in graphic design. Other color naming systemsinclude the RAL Color System, the Toyo Color Finder System and the DICColor System. In other cases, a spot color can be identified usinganother named color system, or by simply specifying a color in adevice-independent color space (e.g., CIELAB). When a spot color isspecified using a color naming system, device-independent color values(e.g., CIELAB values) can be measured and stored in a look-up table foreach of the named spot colors in the color naming system. Deviceindependent color values can then be determined for any objectsspecified by named spot colors by looking up the associated color valuesin the look-up table.

The CIELAB color space is one of the most commonly useddevice-independent color spaces that is used for representing colors incolor printing applications. A color space is said to be“device-independent” when it is tied to the color perceived by a humanobserver rather than to the device coordinates of an imaging device(e.g., RGB or CMYK). As is well-known in the art, the perception of acolor by a human observer can be characterized by the CIE XYZtristimulus values:

X=∫ _(λ) R(λ)I(λ)) x (λ)dλ,

Y=∫ _(λ) R(λ)I(λ)) y (λ)dλ  (1)

Z=∫ _(λ) R(λ)I(λ)) z (λ)dλ,

where R(λ) is the reflection spectrum of the printed color, I(λ) is theillumination spectrum, x(λ), y(×) and Z(λ) are the CIE color matchingfunctions, and the integration is performed over the visible spectrum,which is approximately 400≤λ≤700 nm.

While the CIE XYZ tristimulus values are device-independent, they arenot uniformly related to the human perception of color. The CIELAB colorspace was developed to be a “uniform color space” which implies thatgeometric distances in the color space should be approximatelyproportional to perceived color differences. The CIELAB values arerepresent by three color coordinates L*, a* and b* which can be computedfrom the tristimulus values using the following equations:

L*=116f(Y/Y ₀)−16

a*=500(f(X/X ₀)−f(X/X ₀))  (2)

b*=200(f(Y/Y ₀)−f(Z/Z ₀))

where X₀, Y₀, Z₀ are the tristimulus values of a reference white, and

$\begin{matrix}{{f(t)} = \left\{ \begin{matrix}{\sqrt[3]{t};} & {{{if}t} > 0.008856} \\{{{7.787t} + {16/116}};} & {otherwise}\end{matrix} \right.} & (3)\end{matrix}$

As illustrated in FIG. 4A which shows the CIELAB color space 460, L* isa representation of the lightness of the color, a* is a representationof the greenness-redness of the color, and b* is a representation of theblueness-yellowness of the color. Colors that fall on neutral axis 461(i.e., colors with a*=b*=0) are neutral colors ranging from black (L*=0)to white (L*=100). As illustrated in FIG. 4B, the hue h of a color canbe represented by the angle in the a*-b* plane, and the chroma C* (i.e.,the colorfulness) of a color can be represented by the radius in thea*-b* plane. In equation form:

C*=√{square root over (a* ² +b* ²)}  (4)

h=arctan(b*/a*)

The perceived color difference ΔF* can be approximated by the distancebetween two colors in the CIELAB color space. In equation form:

ΔF*=√{square root over (ΔL* ² +Δa* ² +Δb* ²)}  (5)

Another device-independent color spaces that can be used in someapplications is the well-known CIELUV color space, which represents thecolor in terms of L*, U* and V* color coordinates. The CIECAM02 colorappearance model can also be used to represent color. This model usesmore complex models of the human visual system to compute J, a, b colorvalues that correlate with human color appearance. Correlates of hue hand chroma C can also be calculated. A commonly-used derivative is theCAM02 Uniform Color Space (CAM02-UCS), which is an extension of CIECAM02with tweaks to better match experimental data. Mathematical equationsfor computing color values in any of these color spaces are well-knownto those skilled in the art. It will be obvious that any of these colorspaces, as well as any other device-independent color space known in theart, can be used to represent color in accordance with the presentinvention.

Many commonly used spot colors are outside of the color gamut of typicalprinting systems that utilize CMYK colorants. The color gamut refers tothe set of colors that are reproducible by a printing system, and can bedefined by a volume in a three-dimensional device-independent colorspace such as CIELAB. The color gamut of a printing system will be afunction of the colorants, the receiver media and the print mode. Theouter surface of the color gamut represents the limiting colors that canbe reproduced by the printing system and can be referred to as the gamutsurface. The color gamut of a printing system is commonly represented bystoring a representation of the gamut surface, for example as athree-dimensional object model defined by a mesh of interconnectedpoints in a device-dependent color space. FIG. 5 shows an example of atypical color gamut 500 represented in the CIELAB color space.

In high-volume printing applications, printing systems can be loadedwith specialty colorants (e.g., inks or toners) to accurately reproducethe spot colors. However, this is an impractical solution for low- tomid-volume printing applications since it can be time-consuming andcostly to change the colorants in the printing system. In such cases, itis preferable to compromise on the color accuracy and utilize anin-gamut color that can be printed using the standard colorant eventhough it is necessary to compromise on the color accuracy. The problemthen is to determine the in-gamut color that most closely matches thedesired appearance of the spot color. However, this is not astraight-forward problem to solve given the complex shape of devicecolor gamuts and the subjective preferences of human observers.

An example spot color 502 is illustrated in FIG. 5 , which falls outsidethe color gamut 500 of the printing system. In this example, the spotcolor 502 is a yellow color having a large b* value and a small a*value. In order to reproduce this color using the printing system, it isnecessary to map it to a color which is within the reproducible colorgamut 500. Typically the chosen color will be a color on the color gamutsurface 501. Many “gamut mapping algorithms” exist in the art to performthe mapping of out-of-gamut colors. One such gamut mapping algorithminvolves finding the target color 504 which has the minimum colordifference (e.g., ΔF*) to the spot color 502. While the target color 504may be the closest visual match to the spot color 502, it is frequentlynot the preferred reproduction of the spot color due to the fact that itcan introduce significant hue shifts, particularly when the spot color502 is near one of the sharp “corners” in the color gamut 500corresponding to the printer's primary colors (C, M, Y) or secondarycolors (R, G, B). These hue shifts are often objectionable to a user.For example, if a user specifies that their logo should be printed witha high-chroma lemon yellow spot color 502, they would usually not wantit to be reproduced with a greenish-yellow or orangish-yellow hue, andwould instead want to compromise on the lightness and/or the chroma tokeep the hue closer to the hue of the spot color 502.

Other types of gamut mapping algorithms preserve the hue of the spotcolor 502 to avoid the objectionable hue shift problem associated withthe minimum ΔE* algorithms. One simple algorithm of this type holds thelightness value (L*) and hue value (h) constant and clips the chromavalue (C*) to the color gamut surface 501. The often results in thereproduced color having significantly lower chroma than the spot color502. A more sophisticated hue-preserving gamut mapping algorithminvolves finding the color on the color gamut surface 501 having theminimum color difference (e.g., ΔF*) to the specified spot color 502subject to the constraint that the hue value is equal to the hue valueof the spot color 502. This tends to map the spot color 502 toward thehigh-chroma edges of the color gamut surface 501. FIG. 5 illustrates atarget color 506 determined using a gamut mapping algorithm of thistype.

While the hue-preserving target color 506 will preserve the hue of thespot color 502, it will often have a significantly lower chroma than theminimum color difference target color 504. Often, a particular user maynot prefer either of these two extremes but will instead prefer to mapthe spot color 502 to some other location on the color gamut surface502. And frequently, the preferred color for one user may differ fromthat chosen by a different user. As a result, it is often necessary forusers to perform a time-consuming iterative process to determine thepreferred color reproduction for a particular spot color 501. The methodcommonly involves making repeated adjustments to the target L*a*b*values (or to the printer CMYK values) until the preferred colorreproduction is determined. This typically requires a complex searchprocess given the high-dimensionality of the search space.

The present invention significantly simplifies the process ofdetermining an aim color for reproducing an out-of-gamut spot color byreducing the dimensionality of the search process to provide aone-dimensional control parameter. FIG. 6 shows a flowchart for anexemplary embodiment of the invention. The inputs to the process are aspot color 502 and a color gamut 500

In the following description, the spot color 502 will be referred to asC_(s) and is specified by color coordinates in a three-dimensional colorspace such as CIELAB. The color gamut 500 will likewise be referred toas G_(c), and can be represented by a color gamut surface 501 (FIG. 5 )in the three-dimensional color space representing the colors that can beprinted by the color printing system, which will be referred to asδG_(c). The color gamut 500 is typically determined by printing colorpatches that span the range of input code values for the color channelsof the color printing system. The color of the printed patches are thenmeasured using a device such as a spectrophotometer or a colorimeter,and a color gamut surface 501 that contains all of the printable colorsis determined using a fitting process as is well-known in the art.

If the spot color 502, C_(s), is inside of the color gamut 500, G_(c),(i.e., if C_(s) E G_(c)), then the spot color 502 can be used directlyfor the aim color 560, C_(a), (i.e., C_(a)=C_(s)). Otherwise, the methodof FIG. 6 is used to determine the aim color 560, which will be afunction of the spot color 502, C_(s), and the color gamut 500, G_(c).In the case, the resulting aim color 560 will preferably be a color onthe surface of the color gamut 500 (i.e., C_(a) ∈δG_(c)).

A determine first target color step 520 is used to determine a firsttarget color 504, C_(t1) The quantity C_(t1) is a vector in thethree-dimensional color space (e.g., [L*_(t1), a*_(t1),b*_(t1)]). In apreferred embodiment, the determine first target color step 520determines the first target color 504 by finding the color on thesurface of the color gamut 500 having the minimum color difference(e.g., ΔE*) to the spot color 502. This corresponds to the first targetcolor 504 shown in the example of FIG. 5 .

A determine second target color step 530 is used to determine a secondtarget color 506, C_(t2), having a hue value equal to the hue of thespot color 502. The quantity C_(t2) is a vector in the three-dimensionalcolor space (e.g., [L*_(t2),a*_(t2),b*_(t2)]). In a preferredembodiment, the second target color 506 is the color on the surface ofthe color gamut 500 having the minimum color difference (e.g., ΔE*) tothe specified spot color 502 subject to the constraint that the huevalue is equal to the hue value of the spot color 502. This correspondsto the second target color 506 shown in the example of FIG. 5 . In otherembodiments, other hue preserving gamut mapping methods can be used tofind the second target color 506. For example, the second target color506 can be found by clipping the chroma of the spot color 502 to thesurface of the color gamut 500 while holding the hue and lightnessconstant.

The goal of the invention is to determine a preferred aim color 560 onthe surface of the color gamut 500 for reproducing the spot color 502.The generalized optimization problem would involve searching the entiresurface of the color gamut 500 in the region near the spot color 502.Since the computational complexity of the optimization process increasesexponentially with respect to the dimension of the independent featurespace (i.e., the curse of dimensionality), a constraint is introducedwhich limits the search space to a path 545 specified by aone-dimensional control parameter. In an exemplary embodiment, a definepath step 540 is used to define the path 545 along the surface of thecolor gamut 500 connecting the first target color 504 and the secondtarget color 506 as illustrated in FIG. 7 . A control parameter, a,having a control parameter value 555 is used to specify a relativeposition along the path 545, with α=0 corresponding to the first targetcolor 504 and α=1 corresponding to the second target color 506. The path545 represents a limited search space for determining a preferred aimcolor 560 for reproducing the spot color 502.

The define path step 540 can define the path 545 using a variety ofmethods. In an exemplary embodiment, the control parameter, a, specifiesa hue, h_(a), using a linear relationship:

h _(α) =h ₁+α(h ₂ −h ₁)  (6)

where h₁ is the hue of the first target color 504 and h₂ is the hue ofthe second target color 506. The color C_(α) at a point along the path545 corresponding to a particular control parameter value 555 can thenbe determined by finding the color on the surface of the color gamut 500having the minimum color difference to the specified spot color 502subject to the constraint that the hue value is equal to h_(α).

In another embodiment, the control parameter, a, specifies a relativeposition along a straight line connecting the first target color 504 andthe second target color 506. The color at this position is given by:

C _(α) =C _(t1)+α(C _(t2) −C _(t1))  (7)

The color, c_(α), does not generally lie on the color gamut surface 501.Therefore, the path color, C_(α), can then be found by projecting thecolor c_(α) onto the color gamut surface 501 (for example at a constantlightness and hue).

A determine aim color step 550 is then used to determine aim color 560corresponding a control parameter value 555 which specifies a positionalong the path 545 corresponding to a preferred reproduction of the spotcolor 502. The control parameter value 555 can be determined using avariety of different methods. In an exemplary embodiment, a userinterface 565 is provided which enables a user to adjust the controlparameter value 555 to specify a preferred aim color 560.

In one exemplary embodiment, a test target 600 is printed including aplurality of test patches 605 with sample colors at points along thedefined color path corresponding to a plurality of control parametervalues as illustrated in FIG. 8A. Patch identifiers 610 are included tolabel each of the printed test patches 605. A user interface 565 isprovided that includes features which enables the user to select one ofthe test patches 605 to be used as the aim color 560. The exemplary userinterface 565 includes user instructions 620, patch selection features625 (in this example radio buttons), and a done button 630. In thisembodiment, the number of test patches 605 should be large enough sothat the color difference between the sample colors is relatively small(e.g., about 1-3 ΔE*).

In another exemplary embodiment, the user interface 565 walks the userthrough an iterative process to select the preferred aim color 560. In afirst step, a test target 600 is printed having three or more testpatches 605 as shown in FIG. 8B. The first test patch 605 corresponds tothe first target color 504 (α=0), the last test patch 605 corresponds tothe second target color 506 (α=1), and the intermediate test patches 605correspond to one or more intermediate control parameter values. In theillustrated example, a single intermediate test patch 605 is providedhaving α=0.5. A user interface 565 is provided which enables the user toeither select one of the printed test patches 605 if the user judgesthat it represents a preferred reproduction of the spot color 502, or toselect two of the printed test patches 605 if the user judges that thepreferred reproduction of the spot color 502 falls between them. Toenable these features, the user interface 565 includes exemplary userinterface 565 includes user instructions 620, patch selection features625 (in this example radio buttons), and a done button 630.

If the user selects two of the printed test patches 605, another testtarget 600 is printed having three or more test patches 605, where thefirst and last test patches 605 correspond to the user selected testpatches 605 in the previous iteration, and the intermediate test patches605 correspond to one or more intermediate control parameter values. Forexample, if the user selected the test patches 605 corresponding to α=0and α=0.5, the new test target would have patches with α=0, α=0.25 andα=0.5. The user then uses the user interface 565 as described above toeither select one or two of the printed test patches 605. This processis repeated with increasingly smaller color differences between theprinted test patches 605 until the user indicates that one of theprinted test patches 605 represents a preferred reproduction of the spotcolor 502. While the illustrated example includes only a singleintermediate test patch 605, it will be obvious that a plurality ofintermediate test patches 605 could be used in other embodiments. Thiswould provide smaller color differences between the test patches 605,and would enable the iterative process to converge with feweriterations.

In some embodiments, the path 545 is comprised of colors that fallwithin the color gamut of a soft-copy display. In this case, thepreferred reproduction of the spot color 502 can be selected based onpreviews of the reproduced color presented in an appropriate userinterface 565 such as that shown in FIG. 8C. In this case, the userinterface 565 includes user instructions 620, together with a controlfeature 635 (e.g., a slide bar) that enables the user to adjust thecontrol parameter, and a color patch 640 which displays a preview of thereproduced color corresponding to the control parameter selected usingthe control feature 635. A done button 630 is provided to enable theuser to indicate that the selection process is complete. This method canbe used even if the path 545 includes colors that fall outside the colorgamut of a soft-copy display. However, in this case, any out-of-gamutsample colors must be gamut-mapped to a color that can be displayed onthe soft-copy display. This is preferably done using a hue-preservinggamut mapping algorithm.

The process of determining the preferred aim color 560 for a particularspot color 502 can be a time-consuming labor-intensive process that mustbe repeated for each different spot color 502. However, the optimalcontrol parameter value 555 determined for one spot color 502 should besimilar to the value that would be determined for other similar spotcolors 502. Therefore, to aid in the optimization process a controlparameter prediction function 570 can be determined (e.g., using asupervised learning algorithm) based on the values determined forpreviously evaluated spot colors 502. The inputs to the controlparameter prediction function 570 are the device-independent colorvalues for a spot color, and the output of the control parameterprediction function 570 is a corresponding prediction of the optimalcontrol parameter value 555.

FIG. 9 shows a flowchart of an exemplary method for determining thecontrol parameter prediction function 570. A determine preferred controlparameters step 575 is used to determine a set of control parametervalues 585 corresponding to a set of spot colors 580. The set of spotcolors 580 preferably spans all parts of color space that are likely tobe encountered in spot colors. In an exemplary embodiment, the determinepreferred control parameters step 575 uses the method described withrespect to FIG. 6 to determine the control parameter values 555 for eachof the spot colors 502 in the set of spot colors 580. A fit mathematicalfunction step 590 is used to fit an appropriate mathematical function tothe set of spot colors 580 and the corresponding set of controlparameter values 585 to determine the control parameter predictionfunction 570 having the form:

α=f _(α)(c _(s))  (8)

where α is the predicted control parameter value 555, f_(α)(·) is thecontrol parameter prediction function 570, and C_(s) is the spot color502, which will generally be represented by three coordinates (e.g., L*,a*, b*). Any appropriate form of mathematical function can be used forthe control parameter prediction function 570. For example, the controlparameter prediction function 570 can be a multi-dimensional polynomialfunction, a spline function, a multi-dimensional LUT, and a neuralnetwork function. Preferably, the output of the control parameterprediction function 570 should be constrained to provide predictedcontrol parameter values 555 in the range of 0≤α≤1. In an exemplaryembodiment the control parameter prediction function 570 is a neuralnetwork function where the inputs are the color coordinates of the spotcolor, the substrate and the illumination light source, and the outputis the control parameter value.

In some embodiments, the control parameter prediction function 570 canbe determined by a particular user to represent the color reproductionpreferences of that user. In other embodiments, a set of different userscan each determine a set of color parameter values 585 according totheir preferences, and the determined sets of color parameter values 585can be pooled to determine a single control parameter predictionfunction 570 that is representative of the average preferences.

In some embodiments, each time a user determines a preferred aim color560 for a new spot color 502 using the method of FIG. 6 , the new datacan be added to the set of spot colors 580 and the set of controlparameter values 585, and an updated control parameter predictionfunction 570 can be determined using the method of FIG. 9 .

Generally, the control parameter prediction function 570 will be afunction of the color gamut 500 of the printing system, and thereforedifferent control parameter prediction functions 570 can be determinedfor each printer configuration (e.g., each set of different colorants,media, print modes, etc.). However, in some cases the control parameterprediction functions 570 may be similar enough that a single controlparameter prediction function 570 can be used across at least some ofthe different configurations. Even when it is desirable to providedifferent control parameter prediction functions 570 for differentprinter configurations, it may be appropriate to use the controlparameter prediction function 570 for one configuration as an initialguess at the control parameter prediction function 570 for a newconfiguration. The control parameter prediction function 570 can then berefined as new preferred aim colors are determined.

Once determined, the control parameter prediction function 570 can beused in the method of FIG. 6 to provide an initial guess for the controlparameter value 555 that will produce the preferred reproduction of thespot color 502. The initial guess can be used in any of the methodsdescribed relative to FIGS. 8A-8C. For example, the predicted controlparameter value 555 can be used to create the intermediate test patch605 in first iteration of the method described relative to FIG. 8B, orcan be used as the initial setting of the control feature 635 in themethod described relative to FIG. 8C.

In some cases, the control parameter prediction function 570 can produceadequate reproductions of the spot colors 502 without the need for auser to utilize a manual selection process (such as those describedrelative to FIGS. 8A-8C). In this case, the control parameter predictionfunction 570 can be used to automatically determine the preferred aimcolor 560 for any spot color 502 that comes into the printing system. Insome embodiments, the printing system can provide an option to overridethe automatic aim color selection if a user desires to fine tune thereproduction of a particular spot color 502.

Once the preferred aim color 560 is determined for a spot color 502specified in an input page description file 300 (FIG. 3 ), a colortransform 320 is used to determine the corresponding device coordinates(e.g., CMYK values) that will produce the preferred aim color 560. Forexample, in some embodiments the aim color 560 is specified in terms ofthe CIELAB color space. In this case, the color transform 320 can be aninverse device profile, for example in the format of an ICC profile,which transforms from the CIELAB values to the corresponding CMYKvalues. Once the device coordinates are determined and used by thepre-processing system 305 to provide the corresponding image data 350,the print engine 370 is used to print the image data 350 to form acorresponding printed image 450, wherein the spot colors 502 in the pagedescription file 300 are reproduced as the preferred aim colors 560.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations, combinations, and modifications can be effected by a personof ordinary skill in the art within the spirit and scope of theinvention.

PARTS LIST

-   31 printing module-   32 printing module-   33 printing module-   34 printing module-   35 printing module-   38 print image-   39 fused image-   40 supply unit-   42 receiver-   42 a receiver-   42 b receiver-   50 transfer subsystem-   60 fuser module-   62 fusing roller-   64 pressure roller-   66 fusing nip-   68 release fluid application substation-   69 output tray-   70 finisher-   81 transport web-   86 cleaning station-   99 logic and control unit (LCU)-   100 printer-   111 imaging member-   112 intermediate transfer member-   113 transfer backup member-   201 first transfer nip-   202 second transfer nip-   206 photoreceptor-   210 charging subsystem-   211 meter-   212 meter-   213 grid-   216 surface-   220 exposure subsystem-   225 development station-   226 toning shell-   227 magnetic core-   240 power source-   300 page description file-   305 pre-processing system-   310 digital front end (DFE)-   315 raster image processor (RIP)-   320 color transform processor-   325 compression processor-   330 image processing module-   335 decompression processor-   340 halftone processor-   345 image enhancement processor-   350 image data-   370 print engine-   405 data interface-   430 printer module controller-   435 printer module-   440 image capture system-   450 printed image-   460 CIELAB color space-   461 neutral axis-   462 a*-b* color plane-   464 color coordinate-   500 color gamut-   501 color gamut surface-   502 spot color-   504 target color-   506 target color-   508 neutral axis-   520 determine first target color step-   530 determine second target color step-   540 define path step-   545 path-   550 determine aim color step-   555 control parameter value-   560 aim color-   565 user interface-   570 control parameter prediction function-   575 determine preferred control parameter values step-   580 set of spot colors-   585 set of control parameter values-   590 fit mathematical function step-   600 test target-   605 test patches-   610 patch identifiers-   620 user instructions-   625 patch selection feature-   630 done button-   635 control feature-   640 color patch

1. A method for reproducing an out-of-gamut spot color on a colorprinter, comprising: determining a color gamut for the color printer,the color gamut being defined by a color gamut surface in athree-dimensional color space representing the colors that can beprinted by the color printer; specifying a spot color by colorcoordinates in the three-dimensional color space, wherein the spot coloris outside of the color gamut surface; determining a first target colorcorresponding to a color on the color gamut surface having a minimumcolor difference to the specified spot color; determining a secondtarget color corresponding to a color on the color gamut surface havinga hue value equal to a hue value of the specified spot color; defining apath on the color gamut surface connecting the first target color andthe second target color, wherein a control parameter having a controlparameter value is used to specify a relative position along the definedpath; and providing a user interface enabling a user to adjust thecontrol parameter to specify an aim color along the defined path forreproducing the spot color using the color printer.
 2. The method ofclaim 1, further including reproducing the spot color by printing theselected aim color using the color printer.
 3. The method of claim 1,further including printing sample colors at a plurality of points alongthe defined color path corresponding to a plurality of control parametervalues, and wherein the user interface enables the user to select one ofthe sample colors to be used as the aim color.
 4. The method of claim 1,further including: a) printing sample colors at three or more pointsalong the defined color path corresponding to three or more controlparameter values; b) enabling a user to use the provided user interfaceto either select one of the printed sample colors to be used as the aimcolor or to select two of the sample colors that the aim color liesbetween; c) if the user selects two of the sample colors, enabling aprinting of one or more additional sample colors along the defined colorpath corresponding to one or more control parameter values, wherein theone or more additional sample colors are between the two selected samplecolors; and d) enabling a repeating of steps b) and c) until the userselects one of the printed sample colors to be used as the aim color. 5.The method of claim 1, wherein the user interface is presented on asoft-copy display and includes: a user interface control featureenabling the user to specify a control parameter value corresponding toa relative position along the defined path; a color patch that displaysa preview of a sample color corresponding to the specified controlparameter value; and a user interface selection feature enabling theuser to indicate that the previewed sample color should be used as theaim color.
 6. The method of claim 5, wherein if the sample color isinside a color gamut of the soft-copy display the displayed preview ofthe sample color has the same color as the sample color, and if thesample color is outside the color gamut of the soft-copy display agamut-mapping algorithm is used to determine a color for the displayedpreview of the sample color.
 7. The method of claim 1, wherein the userinterface includes a user interface control feature enabling the user tospecify a control parameter value to select the aim color along thedefined path.
 8. The method of claim 7, further including providing acontrol parameter prediction function which predicts a preferred controlparameter value as a function of the specified color coordinates of thespot color, and wherein the control parameter prediction function isused to determine an initial position of the user interface controlfeature.
 9. The method of claim 8, wherein the control parameterprediction function is determined by enabling one or more observers todetermine preferred control parameter values for a plurality ofdifferent spot colors and fitting a mathematical function to thedetermined preferred control parameter values, wherein inputs to themathematical function are the color coordinates of the spot color and anoutput of the mathematical function is the predicted control parameter.10. The method of claim 1, wherein the control parameter specifies a huevalue between a first hue value corresponding to the first target colorand a second hue value corresponding to the second target color, andwherein a lightness value and a chroma value corresponding to aspecified hue value are determined by finding a color on the color gamutsurface having a minimum color difference to the specified spot colorsubject to the constraint that a hue value of the color is equal to thespecified hue value.
 11. The method of claim 1, wherein the controlparameter specifies an interpolated color between the first target colorand the second target color, wherein a color on the defined pathcorresponding to a particular control parameter is determined byprojecting the interpolated color onto the color gamut surface at aconstant hue and lightness.
 12. The method of claim 1, wherein alightness value and a chroma value for the second target color aredetermined by finding a color on the color gamut surface having aminimum color difference to the specified spot color subject to theconstraint that the hue value is equal to the hue value of the specifiedspot color.
 13. The method of claim 1, wherein the second target colorhas a lightness value equal to the specified spot color.
 14. The methodof claim 1, wherein the three-dimensional color space is CIELAB orCIECAM02 or CAM02-UCS.
 15. The method of claim 1, wherein the colordifference corresponds to a distance in the three-dimensional colorspace.
 16. A method for reproducing an out-of-gamut spot color on acolor printer, comprising: determining a color gamut for the colorprinter, the color gamut being defined by a color gamut surface in athree-dimensional color space representing the colors that can beprinted by the color printer; specifying a spot color by colorcoordinates in the three-dimensional color space, wherein the spot coloris outside of the color gamut surface; determining a first target colorcorresponding to a color on the color gamut surface having a minimumcolor difference to the specified spot color; determining a secondtarget color corresponding to a color on the color gamut surface havinga hue value equal to a hue value of the specified spot color; defining apath on the gamut surface connecting the first target color and thesecond target color, wherein a control parameter having a controlparameter value is used to specify a relative position along the definedpath; and adjusting the control parameter to specify an aim color alongthe defined path for reproducing the spot color using the color printer.